Stock Rom simply refers to the ROM your manufacturer has provided for your device. So yes - the current firmware is your stock-rom. Every official Update from your manufacturer is a new "Stock rom".
– benjaminFeb 18 '16 at 7:32

1 Answer
1

Stock Rom is basically the software which is provided by the vendor or You may can say that the software which was installed by the manufacturer of the phone is Stock Rom. the other type of the Rom is Custom Rom which advance users installed after flashing it as per they desire.

And Yup in your scenario you have Stock Rom of "Lollipop 5.1". You may install any custom Rom as per your own risk. if you know well how to flash custom Rom.